What is claimed is: 1. A method of forming a microstructure in a substrate which comprises: providing a substrate having an exposed surface; and exposing a portion of the exposed surface of the substrate to a light source while providing a chemical atmosphere at the portion of to exposed surface, so that to light causes ablation of the exposed surface of the substrate while the chemical atmosphere effects a change in the chemical functionality of the exposed surface of the substrate in the ablated portion thereof. 2. A method of forming a microstructure in a substrate according to claim 1, wherein the substrate is a member selected from the group consisting of polymeric substrates, glass substrates, silica substrates, ceramic substrates and, composites thereof. 3. A method of forming a microstructure in a substrate according to claim 2, wherein the substrate comprises a polymeric substrate. 4. A method of forming a microstructure in a substrate according to claim 3, wherein the substrate is made from a polymeric material selected from the group consisting of polycarbonate, polystyrene, poly (ethylene terephthalate glycol), poly (methyl methacrylate), and poly (vinyl chloride). 5. A method of forming a microstructure in a substrate according to claim 1, wherein the substrate comprises a microfluidic system. 6. A method of forming a microstructure in a substrate according to claim 1 wherein the chemical atmosphere comprises an inert atmosphere. 7. A method of forming a microstructure in a substrate according to claim 1, wherein the atmosphere comprises a reactive atmosphere. 8. A method of forming a microstructure in a substrate according to claim 7, wherein the atmosphere is selected from the group consisting of oxygen, nitrogen, air, methanol, and water. 9. A method of forming a microstructure in a substrate according to claim 1, wherein the ablation of the substrate forms a channel therein. 10. A method of forming a microstructure in a substrate according to claim 9, wherein the channel is linear. 11. A method of forming a microstructure in a substrate according to claim 9, wherein the channel is non-linear. 12. A method of forming a microstructure in a substrate according to claim 1, wherein said chemical atmosphere is one of a solid, a liquid and a gas. 13. A method of forming a microstructure in a substrate according to claim 1, wherein the change in chemical functionality comprises a change in surface charge. 14. A method of fabricating a microfluidic system having fluid flow channels which comprises: providing a substrate having an exposed surface; and exposing a portion of the exposed surface of the substrate to a light source while providing a chemical atmosphere at the portion of the exposed surface, so that the laser light causes ablation of the exposed surface of to substrate and formation of microchannels therein while the chemical atmosphere effects a change in the chemical functionality of the exposed surface of the substrate in to ablated portion thereof. 15. A method of fabricating a microfluidic system having fluid flow channels according to claim 14, wherein the substrate is a member selected from the group consisting of polymeric substrates, glass substrates, silica substrates, ceramic substrates, and composites thereof. 16. A method of fabricating a microfluidic system having fluid flow channels according to claim 15, wherein the substrate comprises a polymeric substrate. 17. A method of fabricating a microfluidic system having fluid flow channels according to claim 14, wherein the substrate is made from a polymeric material selected from the group consisting of polycarbonate, polystyrene, poly (ethylene terephthalate glycol), poly (methyl methacrylate), and poly (vinyl chloride). 18. A method of fabricating a microfluidic system having fluid flow channels according to claim 14, wherein said chemical atmosphere is one of a solid, a liquid and a gas. 19. A method of fabricating a microfluidic system having fluid flow channels according to claim 14, wherein the chemical atmosphere comprises an inert atmosphere. 20. A method of fabricating a microfluidic system having fluid flow channels according to claim 14, wherein the chemical atmosphere comprises a reactive atmosphere. 21. A method of fabricating a microfluidic system having fluid flow channels according to claim 14, wherein the microchannels comprise at least one of linear and non-linear channels. 22. A method of fabricating a microfluidic system having fluid flow channels according to claim 14, wherein the change in chemical functionality comprises a change in surface charge.